Every Cloudflare installation can secure both your domain and all of its subdomains without additional cost. All you need to do is choose which subdomains to protect with Cloudflare.
For that, after you have activated Cloudflare for your domain, on the same page tick checkboxes of the subdomains and click Save.
If your SSL/Encrypt communication (mode) at Cloudflare Dashboard is set to Full, you need an active SSL certificate for both your domain and subdomains
What to do if I see "Subdomain management has been temporarily disabled"?
If your Cloudflare management page looks like this:
Cloudflare protection temporarily cannot be enabled for your domain. No worries, it will be enabled again in near future!
If having subdomains protected with Cloudflare is crucial for you now, please contact our Customer Success Team - we'll help you out!